In recent years, there has been a renewed interest in renewable energy including solar energy, and this has resulted in extensive research into methods of fabricating higher efficiency solar cells that convert sun light into electricity by the photovoltaic effect. Research continues to design silicon-based solar cells that can achieve higher conversion efficiencies without an exorbitant increase in production costs.
The performance of solar cells and other optoelectronic devices is directly related to optical losses caused by high reflectivity. Flat silicon surfaces such as those found on an untreated silicon wafer have a high natural reflectivity across the entire range of the solar spectrum that could otherwise be converted to electrical energy by the silicon photovoltaic device. To produce high efficiency solar cells, researchers have sought ways to minimize reflection losses. One common approach has been to provide anti-reflection coatings (ARC) that typically are selected based on interference. For example, quarter wavelength transparent layers of materials such as SiOx, TiOx, ZnO, ITO, or Si3N4 are used as ARCs on silicon surfaces. All such ARC coatings are resonant structures and perform well only in a limited spectral range and for specific angles of incidence while the solar spectrum spans a wide range of wavelengths and the incident angle varies during the day. The typical results achieved with simple one-layer ARCs have been a reduction of the surface reflection to about 8 to 15 percent. With more difficult two-layer ARC coatings, the reflectivity can be reduced to about 4 percent, but this kind of coating is expensive to apply and is not effective when placed under glass in photovoltaic modules.
The efficient suppression of reflection in a broad spectral range can be achieved by deep surface texturing. In this regard, etching can be used on a smooth or polished silicon surface to produce rough surfaces with bumps and pits having typical sizes of several or even ten micrometers, and these rough surfaces exhibit reduced reflectivity due to its reflection and absorption characteristics. In one example, anisotropic etching of silicon in KOH/IPA mixtures produces densely packed pyramids that appear black. However, such etching has been typically limited to single crystalline silicon with <1,0,0,> surface orientation, and solar cell design is made more complex by the large penetration pyramids. This texturing also has reflectivity that increases rapidly with the angle of light incidence. More recently, it has been determined that a fine surface texturing on the nanometer scale may be utilized to control reflectivity of silicon surfaces. Specifically, a textured surface with features smaller than the wavelength of light is an effective medium for controlling reflectivity, and testing with regard to solar cell applications has shown that a fine texture that is only about 300 to 500 nanometers in depth and provides a gradual grading of the silicon density and of the index of refraction from the surface to the bulk that is adequate to suppress reflectivity of a silicon surface in the usable spectral range of photon energies above the band gap. Such a textured surface may be thought of a sub-wavelength structured surface that behaves itself as an anti-reflective surface, with the gradually tapered density of the anti-reflective surface suppressing reflection over a wide spectral bandwidth and over a large incidence angle of the incoming light. A method of nanoscale texturing of silicon surfaces has been developed that utilizes wet chemical etching to reduce optical losses due to surface reflection to below 5 percent at all solar wavelengths for crystalline silicon.
The above-described and other reported black silicon solar cells apparently have a zone of high photocarrier recombination throughout the density-graded surface. This high photocarrier recombination layer limits the open-circuit voltage by causing undesirable photocarrier recombination. The high recombination layer further reduces short-circuit current, especially by reducing the collection of photocarriers generated by the blue and green parts of the solar spectrum, e.g., 350 to 700 nanometer wavelength photons (sometimes referred to as “blue response” for the color of light that is in this part of the solar spectrum). Absorption of these blue and green photons occurs within the black silicon layer, and high recombination of created minority-type photocarriers prevents their collection through the circuit and reduces the current produced by the solar cell. This loss of blue response is one of the primary causes of the low efficiencies found in solar cells using black silicon anti-reflection in place of other anti-reflection coatings.
A significant cause of this undesirable high recombination of charge carriers is that the fine nanostructures, especially near the upper surface of the cell, have a tendency during the emitter diffusion to receive too much dopant incorporation. The high level of dopant incorporation leads to defects and thus the formation of so called “dead zone” areas. In these dead zone areas, charges generated recombine before having a chance to contribute significantly to the cell electrical output. Thus, there is a need for diffusion techniques that are both compatible with today's industrial emitter diffusion equipment, but also suited to the porous structures on black silicon surfaces.